Up: Current and Resistance (Ch.
Let us consider an element in an electrical circuit through which a
is flowing. Suppose
there is a potential difference V across this element, and
in a time t a charge Q passes by. The work W
done by the electric field in moving this charge is given by,
W = VQ . Thus, the work done per unit time, or the
power P, is
We recall the units of power are J/s, or Watts (W).
P = = V = VI.
Electrical utilities normally bill on the basis of kilowatt-hours (kWh), which
is the amount of energy the consumer has used in a given time period. One can
convert kWh to J by the following formula:
1 kWh = 1000 Wh = 1000 = 1000 x 3600 = 3.6 x 106 J .